Only four days ago Arsema Dawit was surrounded by those she loved most – her friends, her younger brother and sister and her mother – as she blew out 15 candles on her birthday cake. Yesterday those same friends streamed into the family’s neat three-bedroom flat in Waterloo, South London, struggling to explain why this sweet-natured choirgirl has become the latest victim of London’s knife crime.

Some chose to criticise the Metropolitan Police, which they say failed to take seriously a complaint that her alleged murderer had harassed and assaulted her in the weeks leading up to her death. But most were too overcome with grief to speak.

Women covered their heads with white shawls out of respect for the family, some wailing as they entered the building. Arsema’s mother, Tsehay, surrounded by women, sat in one room of the flat with her son Robel, 13, and daughter Feruz, 12.

She was too distraught to speak to visitors, able only to acknowledge them with a nod. Berekati Asfeday, a family friend, said that Arsema’s mother was so upset that she “can’t talk, just cry, cry, cry, non-stop cry”.

The men sat in a separate room of the family home, which was was decorated with religious icons. “Everyone is so upset. It is a deep grief,” one male visitor said.

Those who had the strength to talk described a happy and deeply religious teenager who had fought against the odds since arriving in Britain from Eritrea more than four years ago.

The family had moved to at least three homes across South London before settling within sight of the Houses of Parliament. Friends said that the children’s father had not lived with the family for some time.

Arsema had learnt English and was busy preparing for her GCSEs at the nearby Harris Academy, with dreams of university and a career.

Simon Tesfaghiorgish, a family friend, said: “Arsema’s life was maths, history, reading and reading the Bible.”

She was determined not to forget her African heritage and was an enthusiastic member of the Eritrean community church, singing in its 20-strong choir. Most members of the congregation at St Michael’s Community Church in Camberwell were Eritreans fleeing the fighting against Ethiopian soldiers in the East African region.

Father Yohannes Sibhatu, the parish priest, said: “She worshipped in this church and used to sing in the choir. She used to be very active, but in the last two years, for some reason, she was quite quiet.

“She was a nice girl, very likeable, and she may have had quite a few friends in the church. I am really shocked and really sorry for the family that this has happened. As a priest and as a member of the community – like everyone else – I am feeling very sad.”

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